The term "pultrusion" is generally used to mean the art for manufacturing FRP wherein a continuous reinforcing material, most commonly, a filamentary material, is first impregnated with a curable or polymerizable liquid resin and pulled through a die having a desired cross section to shape the impregnated resinous material with polymerization cure into a cured product having a continuous length having a uniform cross section which has been molded by the die.
In pultrusion, because any polymerizable or curable liquid resin may be utilized, polymerization or cure by radiation of ultraviolet ray, which will be noted UV, has conventionally been studied. However, a difficulty involved in a curing process by UV radiation lay in strong sticking trend of uncured or half-cured liquid resin onto the die inside surfaces, with which the resin is in contact during the process from initial liquid stage to cured solid stage. Specifically, a commonly applied die was a pipe made of glass, which is not absorptive of UV and, through the glass wall externally, UV was radiated at internally travelling target things to be cured. In such a process, a sticking or adhering trend of the resin onto the internal face of the glass pipe was inevitable which demands more powerful pull force and, even if such an operation were maintained with overcoming force, it was not possible to avoid occurrence of deposit or layering of polymerized resin thereon with time, which would impede exact regeneration products of an applied die section shape, thus commonly rendering thinner or deformed cross sections.